1. Field of the Invention
The invention relates to a method and apparatus for effecting the press-fit assemblage of a generally cylindrical strain sensor in, for example, an erected steel column or other structural element in which in situ stress measurements are desired. In accomplishing this objective, the invention also provides a method and apparatus for effecting the drilling and finishing of a hole in a structural element to precise dimensions through the utilization of hand-held tools.
2. Background of the Invention
Strain sensor elements capable of developing an electrical signal when subjected to compressive or tensile stress have long been known in the art. In recent years, such strain sensor elements have been incorporated in generally cylindrical metallic sheets which are then press fitted into a hole provided in a portion of a metallic structural element in which stress determinations are desired. The press fit is necessary in order that the stress existing in the structural element will be transmitted to the strain sensors, which are mounted within the interior of the cylindrical element. A typical strain gauge construction of this type is shown in U.S. Pat. No. 4,530,245 to JACOBSON.
If the cylindrical strain gauge is assembled in the structural steel element prior to the incorporation of the structural steel element in a building or other structure, there is no particular problem involved in utilizing existing types of precision boring or drilling machines for fabricating a precise hole through the portion of the structural steel element wherein stress measurements are desired. Once, however, the structural steel element is erected, it is a matter of some difficulty to produce by hand-held tools, a bore through any portion of an erected structural steel element which will be sufficiently true and of the proper diameter to ensure that a snug press-fit of the cylindrical strain gauge element can be effected. Additionally, the press-fit insertion of the cylindrical strain gauge element into any such hole presents a second problem because the sensor element is of fairly delicate construction and must be protected from abuse. Driving the cylindrical shell into the hole with a hammer would not be a satisfactory method of press-fit insertion. The insertion problem is further complicated by the fact that such cylindrical strain gauge element inherently has one or more pairs of electrical conductors coming out of the element to transmit the electrical signals generated by the strain gauge sensor elements to suitable amplifying, indicating or recording devices.
The prior art has not provided a satisfactory solution for effecting the drilling of a precise hole in an installed structural steel element, and then press-fitting a cylindrical strain gauge sensor in such hole without damage and solely through the utilization of hand-held tools.